If using shadow passwords, you need to be root because you have to verify passwords against /etc/shadow. Then you can run through /etc/passwd. Might be best to do with a C program - doing it in a script could be a bit of an ask

It could be a risk running a script or command with username/password as an argument.
If someone makes an '$>ps -ef' during this short time they will probably see a password or worse, username and password.

I wish I had and waterproof answer but at the moment I don't.
At least you could run these tests on a closed machine where only you have access?

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